Fiber optic cables are terminated with polished connectors that interchangeably interconnect with low optical insertion loss to other patchcords or fiber optic devices having compatible connectors. These connectors include an optical fiber, one end of which is stripped to expose the bare glass and bonded within a precision, close tolerance hole of a ferrule. The fiber and ferrule end faces are made co-planar and optically smooth by cleaving or subsequent polishing of the end face. In the common male-type fiber optic termination, a length of polished ferrule containing the optical fiber extends outside of the connector housing.
Male-type connectorized fibers may be interconnected to one another with low optical loss (<0.25 dB) in transmission by inserting the connectors into opposite ends of a fiber optic union adapter. Union adapters typically consist of a housing with opposing receptacles that surround a hollow, precision split sleeve whose nominal inner diameter is slightly less than the outer diameter of connectors' ferrules. The mating of the ferrules within the union adapter elastically deforms the semi-tubular wall of the split sleeve to slightly enlarge the inner diameter of the sleeve. The sleeve produces an opposing compressive force on the ferrules which aligns the ferrules concentrically. Precision manufacturing ensures that the optical fiber core is concentric with the optical fiber outer diameter, and the hole within the ferrule is concentric with the ferrule outer diameter at one end of the ferrule. Consequently, the two fiber cores are repeatedly aligned concentrically to micron or sub-micron tolerances. A slight axial force on the ferrules is produced once the spring-loaded bodies of the connector assemblies are attached to the housing of the union adapter, ensuring that the domed, polished end faces of the fiber/ferrule assemblies of the two different cables are mechanically and optically contacted within the split sleeve.
The polished ferrule contact areas are highly susceptible to scratching caused by repeated mating and demating cycles in the presence of contaminants trapped on or in the vicinity of the contact area. Surface damage to the fiber endface in the vicinity of the optical fiber's core degrades optical performance. In particular, the increased excess loss and reduced return loss can seriously compromise the network's performance. With broadcast-type access networks, in which the optical signal is power split between as many as thirty-two users, the optical power budget of the network has low margin and the impact of such damage is particularly significant. This problem is exacerbated by the fact that a single contaminated or damaged fiber/ferrule, if connected to other clean and undamaged fiber terminations, can degrade these other fiber terminations and propagate connector damage throughout the network.
In the past, the primary users of fiber optic telecommunications equipment have been service providers such as telephony and cable operators delivering data, video and telephone transmission. Their optical networking equipment has historically been centrally located within specialized facilities maintained and operated by highly experienced engineers. A growth in applications of fiber optic technology is occurring as fiber is increasingly being deployed in local area networks (LANs) located in the end users' facilities. In this decentralized architecture, the cost to diagnose and repair damaged terminations increases considerably depending on the physical location of the termination within the network. For instance, damage to an inaccessible connectorized drop cable originating from within a customer's wall or damage at the connector interface of a populated, high-density fiber patch panel requires a costly service call and repair by an experienced technician. These are two examples of “back-side” fiber optic terminations which are difficult to repair by virtue of their inaccessibility.
Fiber optic access networks may incorporate large numbers of reconfigurable connection interfaces as the fibers branch out from a central closet to each access location. For instance, fiber optic patch cables attach at one end to connectors at wall or desk mount interface plate and at the other end to fiber optic modems or gigabit Ethernet transceivers. Typically, the ends of the fiber optic drop cable within the customer's premises are terminated using highly specialized and costly fiber optic termination equipment. Once the fiber build-out is complete, proper handling of the fiber cable and connectors must be diligently maintained to preserve the performance of the network. Fiber optic cable is particularly susceptible to cracking due to excessive bends and polished fiber optic terminations are susceptible to scratching if contacted with dirty and contaminated connectors. Repair and debugging requires skilled fiber optic technicians, adding significant cost and overhead to maintain the network. As a consequence, present day fiber optic systems lack the robustness commonly found in electronic networking systems.
Recent advances in the design of union adapters for various standard connector styles (FC, SC, ST, LC, MTRJ) have focused on approaches to prevent contamination from entering the critical split sleeve area. This includes the development of various shields and covers to help prevent contamination from entering the front side union adaptor body. U.S. Pat. No. 5,887,098 by Ernst et al. discloses an FC-type fiber optic union adapter with a two-part shield assembly to cover the end of the receptacle when a cable is not attached. U.S. Pat. No. 6,863,445 by Ngo describes an alternate cap design for SC type fiber optic union adapters. However, these approaches do not prevent a damaged or contaminated connector ferrule from damaging the mating connector.
In addition, an alternate type of fiber optic adapter is designed to produce substantial signal attenuation by introducing an air gap or misalignment between opposing connector ferrules or by inserting a lossy optical element between the mating ferrules are available. For example, U.S. Patent Application 2003/031423 by Zimmel describes an SC-type fiber optic adapter that includes a sheet of attenuator glass embedded at the longitudinal center of the alignment split sleeve and U.S. Pat. No. 5,267,342 by Takahashi et al. introduces an air gap between connector ferrules to cause light to escape from the central waveguide. This adapter produces significant insertion loss (>=5 dB) since it is designed to produce attenuation. These attenuators interrupt the longitudinal continuity of the central waveguide cores attached to either side of the attenuator housing and thereby introduce a significant amount of loss and optical backreflection. These devices rely on a non-adiabatic or abrupt discontinuity in the waveguide core as is passes through the attenuating union adapter.
A low loss, low backreflection, low cost and compact device to prevent polished surface damage (PSD) from propagating to other fiber optic connectors and fiber optic devices is therefore of particular importance, much like its analog, the electrical fuse, which is also a sacrificial element protecting costly electronic systems from damage and which can be inexpensively and easily replaced.